Digital light processing (DLP) is of interest for projection display systems, such as projecting images in conference rooms, home television systems, advertising displays, automobile dashboard and heads-up displays and other applications. In a DLP system a spatial light modulator modulates a light source in order to generate pixels at a projection surface with controlled intensity. A light valve is a type of spatial light modulator that modulates light across an array of elements. A light valve typically modulates optical transmission or reflection properties across an array. For example, a reflective liquid crystal light valves utilize an array of liquid crystals elements to modulate the intensity of reflected light across the array. Another common type of light valve is a digital micro-mirror device chip (often known as a “digital light valve”) that has an array of movable micro-mirrors that can be individually tilted between two positions to vary the amount of light per-pixel that is reflected onto a display surface.
Cost considerations typically require that a single spatial modulator be used in a DLP system. Conventionally a bright white light is used as the light source. A rotating color wheel is used to separate out red, green, and blue light from the white light source. Thus, when the red filter of the color wheel is aligned to the white light source, red light is focused onto the spatial modulator for the red color of pixels, and so one for the green and blue filters of the color wheel.
There are several drawbacks to conventional DLP systems. First, the image is sometimes not as bright as desired. Conventional white light sources produce a limited number of lumens of light. Additionally, conventional DLP systems waste a considerable amount of the light energy. Second, some attributes of the displayed image, such as color saturation, are deleteriously affected by the color wheel, which can introduce artifacts into the displayed image. Third, DLP systems include expensive optical elements.